Diamond is a material having excellent properties as a semiconductor. Additionally, diamond is expected to be applied to electronic devices unfeasible with existing semiconductor materials. Examples of such electronic devices include high frequency devices, power devices and ultraviolet light emitting devices. Further, diamond is a material having a negative electron affinity (NEA), and hence is expected to be applied to electron emitting devices to operate at low voltages.
However, when diamond is applied as electronic devices, defects contained in the crystal affects the properties of the devices, and hence there is needed a diamond that contains as few crystal defects as possible and has a crystallinity close to the crystallinity of a single crystal. Further, for the purpose of industrialization of electronic devices using diamond, it is essential to place the production of diamond into the production line of a factory. Therefore, needed is a technique for synthesizing a high quality and large area diamond film.
In this connection, as a general synthesis method of diamond, known is a high pressure high temperature (HPHT) synthesis method; according to this method, a high quality diamond smaller in the content of crystal defects than natural diamond can be synthesized depending on the conditions. The size of the largest hitherto reported diamond based on the HPHT method is approximately 10×10 mm. As another method for synthesizing diamond, there is a chemical vapor deposition (CVD) method. This synthesis method is a film formation method in which an introduced source gas is decomposed by means of a method of some kind and thus, a desired material is deposited on a substrate. Therefore, according to this synthesis method, the size of the substrate is not limited in principle, and hence this method can be expected to be applied as a method for forming a diamond film on a large area substrate. For example, either the microwave plasma CVD method or the direct current plasma CVD method is considered to be capable of forming a film of polycrystalline diamond on a substrate of 4 inches (approximately 10 cm) or more in diameter.
Additionally, for the purpose of obtaining a large size single crystal diamond, an attempt has been made in which diamond is epitaxially grown on a foreign substrate. Examples of the substrate materials for which epitaxial diamond growth has hitherto been identified include, cubic boron nitride (c-BN) [see Non-patent Document 1], nickel (Ni), silicon (Si) [see Non-patent Document 3], zinc-blende type silicon carbide (β-SiC) [see Non-patent Document 4], cobalt (Co), platinum (Pt) [see Non-patent Document 5] and iridium (Ir). Among these, only iridium enables synthesis of a large area single crystal and growth of a high quality diamond which does not include carbon components other than diamond and does not include non-epitaxial diamond particles involving rotation or inclination. Accordingly, use of iridium for an underlayer of epitaxial diamond may offer a possibility of obtaining a large area, high quality diamond.
For the purpose of obtaining a large area diamond through hetero-epitaxial growth, it is essential to prepare a base material so as to have a large area. Iridium permits preparing an epitaxial thin film by sputtering or vacuum deposition. However, important is an investigation of base materials for use in growth of iridium for a large area formation. For the substrate for growth of iridium, magnesium oxide (MgO), SrTiO3 (STO) and sapphire (α-Al2O3) have hitherto been used.
For growth of epitaxial diamond on iridium, known is the pretreatment of the base surface for the purpose of bias enhanced nucleation [see Non-patent Document 2]. In other words, in the bias enhanced nucleation, by exposing an ion-containing plasma to the iridium base surface, epitaxial diamond nuclei are formed. Successive application of the CVD method to a long time diamond growth enables a preparation of a free-standing epitaxial diamond film. Examples of the bias enhanced nucleation apparatus usable for applying such a pretreatment as described above include a microwave plasma CVD apparatus, a three electrode direct current plasma CVD apparatus [see Non-patent Document 6] and a parallel plate electrode type direct current plasma generator.
The parallel plate electrode type direct current plasma generator is an apparatus developed for solving a problem associated with the diamond nucleation on an iridium base with the three electrode direct current plasma CVD apparatus. The problem concerned is such that the nuclei are generated non-uniformly, and accordingly, the diamond formed on the substrate is divided into an epitaxial growth area, a non-epitaxial growth area and a non-growth area. The anode of the three electrode direct current plasma CVD apparatus is of a ring shape, and this shape is probably the cause for the non-uniform nucleation. Accordingly, in the parallel plate electrode type direct current plasma generator, adoption of a flat plate anode has enabled an extension of the epitaxial growth area of diamond over the whole surface of, for example, an 10×10 mm iridium base.
However, adoption of such a large area of iridium base in such a parallel plate electrode type direct current plasma generator necessitates a size and shape change of the cathode; thus, essential is a development of an appropriate anode diameter and the separation between the anode and the substrate (hereinafter, these are collectively referred to as the electrode layout) to be compatible with such a change as described above. Additionally, a large area iridium base may be coped with discharge current increase with a fixed current density; however, such a discharge current increase is anticipated to cause an increase of the heat amount generated in the substrate and an increase of the substrate temperature. In this connection, the substrate temperature at the time of the bias enhanced nucleation is a parameter to give a remarkable change to the number density of diamond particles, and hence there are various subjects to be developed such as the substrate temperature required to be controlled independently of the discharge current.    Non-patent Document 1: S. Koizumi, T. Murakami, K. Suzuki and T. Inuzuka, Appl. Phys. Lett., Vol. 57, No. 6, pp. 563-565 (1990)    Non-patent Document 2: S. Yugo, T. Kanai, T. Kimura and T. Muto, Appl. Phys. Lett., Vol. 58, No. 10, pp. 1036-1038 (1991)    Non-patent Document 3: B. R. Stoner and J. T. Glass, Appl. Phys. Lett., Vol. 60, No. 6, PP. 698-700 (1992)    Non-patent Document 4: P. C. Yang, W. Zhu and J. T. Glass, J. Mater. Res., Vol. 8, No. 8, pp. 1773-1776 (1993)    Non-patent Document 5: T. Tachibana, Y. Yokota, K. Miyata, K. Kobashi and Y. Shintani, Diamond and Related Materials, Vol. 6, Nos. 2-4, pp. 266-271 (1997)    Non-patent Document 6: K. Ohtsuka, K. Suzuki, A. Sawabe and T. Inuzuka, Jpn. J. Appl. Phys., Vol. 35, No. 8B, pp. L1072-1074 (1996)